1. Field of the Invention
The present invention relates to a bipolar transistor and, more particularly, to a bipolar transistor with a box-type germanium profile that lies outside of the emitter-base depletion region.
2. Description of the Related Art
A bipolar transistor is a three-terminal device that can, when properly biased, controllably vary the magnitude of the current that flows between two of the terminals. The three terminals include a base terminal, a collector terminal, and an emitter terminal. The charge carriers, which form the current, flow from the emitter terminal to the collector terminal, while variations in the voltage on the base terminal cause the magnitude of the current to vary.
FIG. 1 shows a cross-sectional diagram that illustrates a portion of a prior-art bipolar transistor 100. As shown in FIG. 1, transistor 100 includes an n- epitaxial layer 110 that functions as the collector, shallow trench isolation (STI) regions 112 that are formed in layer 110, and an epitaxial layer 114 that is formed on layer 110 and STI regions 112.
Epitaxial layer 114 includes a p-type doped region 116 that has a lightly-doped region 116A, and a heavily-doped region 116B that is formed below lightly-doped region 116A. Lightly-doped region 116A has a top surface and a bottom region, while heavily-doped region 116B has a top region and a bottom surface. The bottom region of lightly-doped region 116A and the top region of heavily-doped region 116B form a transition region where the dopant concentration increases from the concentration of lightly-doped region 116A to the concentration of heavily-doped region 116B.
Epitaxial layer 114 also includes an n-type doped region 118 that is formed in p-type doped region 116. When unbiased, the physical junction between p-type doped region 116 and n-type doped region 118 forms a depletion region 120 where a junction region of doped region 116A is free of holes and a junction region of doped region 118 is free of electrons. When doped region 116 and doped region 118 are forward biased, a smaller depletion region 120A is formed. (A larger depletion region is formed when the doped region 116 and doped region 118 are reverse biased.)
As further shown in FIG. 1, epitaxial layer 114 includes a silicon germanium (SiGe) layer 114A with a top surface that is positioned to lie within the smaller forward biased depletion region 120A. In addition, the peripheral regions of SiGe layer 114A have a p+ dopant concentration. Epitaxial layer 114 further includes a cap layer 114B that is formed on SiGe layer 114A to have substantially no germanium.
Transistor 100 also includes an n+ polysilicon (poly) region 122 that is formed on cap layer 114B, and an oxide layer 124 that contacts cap layer 114B and poly region 122. Doped region 118 and poly region 122 function as the emitter of transistor 100, while doped region 116 functions as the base.
Transistor 100 additionally includes a nitride layer 126 that contacts oxide layer 124 and poly emitter 122, and a metalization plug 128 that is formed on poly emitter 122. In addition, a base contact 130 is formed on SiGe layer 114A to contact the peripheral p+ region of SiGe layer 114A.
FIG. 2 shows a graph that illustrates a vertical view of epitaxial layer 114 under the central portion of poly region 122. As shown in FIG. 2, n-type doped region 118 of epitaxial layer 114 extends down from the top surface of epitaxial layer 114 to a depth D1. Doped region 118 has an n+ dopant concentration at the surface that decreases to substantially zero at depth D1.
In addition, p-type doped region 116 of epitaxial layer 114 extends down from depth D1 to a depth D2. (Epitaxial layer 114 also includes n-type dopants that outdiffused from the collector that extend from a depth D3 to the bottom surface.) Doped region 116 includes lightly-doped region 116A that lies adjacent to doped region 118, and heavily-doped region 116B that is formed under lightly-doped region 116A.
In addition, as further shown in FIG. 2, SiGe layer 114A has a substantially uniform concentration of germanium (approximately 0.15 mole fraction) throughout layer 114A. A SiGe layer with a substantially constant concentration of germanium is known as having a box-type germanium profile.
A box-type germanium profile has a down ramp 132 that defines a thin top surface layer 134 of SiGe layer 114A that has a decreasing germanium concentration. (The germanium concentration can not go to zero instantly.) Thus, down ramp 132 is substantially linear with a very large slope.
One important measure of a transistor is the amount of base to collector current amplification, known, as beta, provided by the transistor. The germanium in epitaxial layer 114 increases the beta when compared to a standard silicon base layer that has no germanium. This is because SiGe layer 114A has a narrower band gap than the silicon of doped region 118. The difference in band gaps enhances the efficiency of carrier injection from doped region 118 into doped region 116.
To obtain the increased beta, surface layer 134 of SiGe layer 114A must lie within the smaller depletion region 120A when transistor 100 is forward biased. When surface layer 134 of SiGe layer 114A does not lie within depletion region 120A when forward biased, there is substantially no increase in the beta of transistor 100 over what can be obtained with a comparable transistor that does not utilize a silicon germanium base. Thus, if cap layer 114B is too thick and surface layer 134 of SiGe layer 114A does not lie within junction depletion region 120A when forward biased, SiGe layer 114A provides no appreciable increase in beta.
Another important measure of a bipolar transistor is the time required for a minority carrier to pass through the base region, known as the base transit time. This is an important measure for gigahertz frequency devices because the base transit time is one of the major components of the total transit time which, in turn, limits the maximum frequency of the signal.
One component of the base transit time is the boron concentration in doped region 116. As noted above, doped region 116 (the base) has a lightly doped region 116A adjacent to doped region 118 (the emitter) and a heavily doped region 116B adjacent to lightly doped region 116A.
As a result, the boron concentration decreases from a peak value at a depth D4 in heavily doped base region 116B when moving towards doped region 118 (the emitter). This decrease in boron concentration sets up a retarding field that slows electrons injected by the emitter. This results in increased base transit time and, as a result, a decreased fT peak.
One approach to reducing the base transit time is to utilize a SiGe layer with a triangular-type germanium profile. FIG. 3 shows a graph that illustrates a transistor 300 with a triangular-type germanium profile. Transistor 300 is similar to transistor 100 and, as a result, utilizes the same reference numerals to designate the structures that are common to both transistors.
As shown in FIG. 3, transistor 300 differs from transistor 100 in that transistor 300 has a triangular-type germanium profile with a down ramp 310 that extends from the top surface of SiGe layer 314A at depth D1 to a point at depth D5 that corresponds with the location of the peak germanium concentration. As a result, the concentration of germanium linearly increases from zero at the top surface of SiGe layer 314A to approximately 0.15 mole fraction at depth D5 near the bottom surface of SiGe layer 314A with a slope that is much less than the slope of down ramp 132.
In operation, the decreasing-towards-the-emitter slope of the triangular-type germanium profile sets up a uniform field across SiGe layer 314A that is opposite to the retarding field set up by the decreasing-towards-the-emitter slope of the boron dopant concentration. As a result, electrons injected by the emitter (doped region 118) experience a reduced retarding field and, therefore, have a reduced base transit time. However, transistors with triangular-type germanium profiles have a number of disadvantages including being difficult and expensive to fabricate.
In addition to box-type and triangular-type germanium profiles, other types of profiles have also been used. For example, a silicon germanium block with a lower concentration pedestal formed on the block is described by Deixler et al., xe2x80x9cExplorations for High Performance SiGe-Heterojunction Bipolar Transistor Integration,xe2x80x9d IEEE Proc. BCTM 1.3, pp. 30-33, 2001.
The Deixler germanium profile has a number of advantages over a standard box-type germanium profile including improved base current linearity, decreased emitter base capacitance, and increased fT at low current levels. However, the Deixler profile has one significant drawback, that being that the Deixler profile also has a decreasing-towards-the-emitter boron concentration.
As a result, electrons injected by the emitter into the base also experience the retarding field caused by the decreasing-towards-the-emitter boron concentration. This results in increased base transit time and, as a result, a decreased fT peak. Thus, there is a need for alternate structures that can reduce the base transit time of a bipolar transistor.
The present invention provides a bipolar transistor that has a boxed germanium profile with a reduced transit time. The bipolar transistor of the present invention includes a first epitaxial layer of a first conductivity type, and a second epitaxial layer that contacts the first epitaxial layer.
The second epitaxial layer has a first doped region of the first conductivity type, and a second doped region of a second conductivity type that is formed below the first doped region. The second doped region has a lightly-doped region adjacent to the first doped region, and a heavily-doped region adjacent to the lightly-doped region. The lightly-doped region has a top surface, and the heavily-doped region has a peak dopant concentration at a peak position within the heavily-doped region.
The second epitaxial layer also includes a depletion region between the first doped region and the second doped region, and a silicon germanium (SiGe) region that has a top surface. The top surface of the SiGe region is spaced apart from the depletion region. Further, the transistor of the present invention has a conductive region that is formed to contact the first doped region.
In addition, the top surface of the SiGe region can lie within a range from below the top surface of the lightly-doped region, and to above the peak position. For example, the top surface of the SiGe region can lie in a transition region between the lightly-doped region and the heavily-doped region at a point where the dopant concentration has the largest gradient.